1. Field of the Invention
The invention relates to devices and methods for the acquisition of mass spectra of ions separated by their mobility.
2. Description of the Related Art
Mass spectrometers can only ever determine the ratio of the ion mass m to the number z of charges of the ion. Where the terms “mass of an ion” or “ion mass” are used below for simplification, they always refer to the mass m of the ion, divided by the dimensionless number z of elementary charges. “Ion species” shall denote ions having the same elemental composition, the same charge and the same three-dimensional structure. The ion species generally comprise all ions of an isotope group, which consist of ions of slightly different masses, but virtually the same mobilities. “Isomers” refer to molecules with the same elemental composition but different spatial arrangements of the elements or groups of elements within the molecule.
Particularly for bioorganic molecules, knowledge about the different kinds of isomers becomes more and more essential: isomers related to the primary structure (structural isomers) and particularly isomers related to the secondary or tertiary structure (conformational isomers). All Isomers have different geometrical forms but exactly the same mass. It is therefore impossible to differentiate between isomers on the basis of their mass. Some information as to the structure can be obtained from fragment ion mass spectra; however, an efficient and certain way to recognize, distinguish, and select such isomers is to separate their ions according to their different mobilities.
The mobility of ions can be measured via their drift velocities in a gas under the influence of an electric field. Either a drift region is filled with an inert gas such as helium, nitrogen or argon at rest, and the ions of the substance under investigation are pulled through the resting gas by means of a (mostly homogeneous) electric field, or the ions are blown by a gas against a spatially increasing electrical field (a “field barrier”), where the ions assume equilibrium positions along the rising field according to their mobility. In a resting gas and under a constant electric field strength E, the drift velocity vd of ions in the gas is proportional to the electric field strength E: vd=K×E. The proportionality factor K is called the “ion mobility coefficient” (or simply “ion mobility”) of the ion species. The ion mobility K is particularly a function of the collision cross-section of the ions, but influenced by the gas temperature, gas pressure, gas speed, type of gas, and ion charge.
A number of academic research groups have coupled ion mobility spectrometers that use long drift tubes with mass spectrometers. A pressure range of several hectopascals has been adopted almost universally for the mobility drift region. For achieving high mobility resolution, drift regions with several meters are necessary, and electric field strengths of 2,000 volts per meter and more are applied. The ions are usually introduced into the drift region in the form of short ion pulses by a gating device, producing spatially small ion clouds, which are pulled through the drift region by the electric field. In the gas of the drift region, these ion clouds are subject to diffusion. The diffusion acts equally in all directions, radially as well as axially, limiting the mobility resolution Rmob=K/ΔK=vd/Δvd, where ΔK is the width of the ion signal of the mobility Kat half height, and Δvd is the correspondent difference in speed. Mobility values above Rmob=60 can be regarded as “high resolution mobility”. In the best cases, mobility resolutions up to Rmob=200 have been achieved in long drift tube instruments.
High-resolution time-of-flight mass spectrometers with orthogonal injection of the ions (OTOF-MS), in particular, have proven successful for combinations of mobility spectrometers with mass spectrometers. For such combinations, the high-resolution ion mobility spectrometers of the current type have the disadvantage of being several meters long. Such a solution is unfavorable for instruments marketed commercially. Even ion mobility spectrometers with a straight drift region offering only moderate resolution are about one meter long. For the construction of small, high-resolution mobility analyzers, one therefore has to look for a solution which shortens the overall length but does not diminish the mobility resolution.
In document U.S. Pat. No. 7,838,826 B1 (M. A. Park, 2008), an ion mobility spectrometer is presented, the length of which amounts to about five centimeters only. It is based upon a moving gas which drives ions against and over an electric counter-field barrier in a special ion tunnel at the exit of a modified ion funnel built into an orthogonal time-of-flight mass spectrometer. Unlike many other trials to build small ion mobility spectrometers, the small device by M. A. Park has already achieved ion mobility resolutions up to Rmob=250.
The apparatus of M. A. Park and its operation are schematically illustrated in FIGS. 1 and 2. In the bottom part of FIG. 1, the basic design of the device is presented, with an entrance funnel (10) and an exit funnel (12), both with openings between the electrodes to let escape the gas. Between the two funnels (10, 12), a closed tube-like quadrupole tunnel (11) is formed by thin electrodes and arranged along the z-axis. The electrodes are separated from each other by insulating material closing the gaps between the electrodes forming a circular tube. At the top of FIG. 1, the electrodes (15, 16) of the funnel (10) and the electrodes (17, 18) of the quadrupole tunnel (11) are shown. They are segmented into quadrants to allow for the generation of a quadrupolar RF electric field inside. The electrodes of the tube-like tunnel (11) are shown with equipotential lines of the quadrupolar RF field inside the tube at a given time. The differential pumping system of the mass spectrometer, surrounding the ion mobility spectrometer, is dimensioned to cause a gas to flow through the tube-like tunnel (11) in a laminar way, thereby approximating a parabolic velocity profile (14). Ions entering the first funnel (10) are entrained by the gas and collisionally focused onto the axis of the tube-like tunnel (11) under the effect of the pseudo-potential. They move, driven by the gas, along the axis of the tube-like tunnel (11) towards its exit through the apertured diaphragm (13). Most of the gas escapes through gaps between the electrodes of the second funnel (12).
Within the tube-like tunnel (11), an electric DC field barrier stops the ions and spatially separates the ions by their mobilities.
An ion funnel is usually operated with apertured diaphragms the opening of which tapers to smaller diameters thus forming an inner volume in the shape of a funnel. The two phases of an RF voltage are applied alternately to the diaphragms to build up a pseudopotential which keeps the ions away from the funnel walls. A DC potential gradient at the diaphragms drives the ions to and through the narrow end of the funnel. The first (entrance) ion funnel (10) is here built from electrodes which are divided into four parts to allow a more sophisticated RF field.
FIG. 2 outlines the operation of this device. Entrained by a gas (7), ions from an electrospray ion source (not shown) are introduced via capillary (8) into the first chamber of a vacuum system. A repelling DC potential applied to the repeller plate (9) drives the ions (6) into the entrance funnel (10) of the mobility spectrometer. The entrance funnel (10) guides the ions into the tube-like tunnel (11), where the ions are driven by the gas flow (14) against an electric DC field barrier. In the bottom part of FIG. 2, three phases of the profile of the electric DC field barrier are shown. Between z-axis locations (20) and (23), the axial electric field increases linearly, generated by a quadratically increasing electric potential. Between z-axis locations (23) and (24), the field remains substantially constant, forming a plateau of the electric DC field barrier, generated by a linear increase of the electrical potential. In a simple device, the electric field profile can be generated by a single voltage, which is applied to the diaphragm electrode at location (24) and divided by precision resistors along the diaphragm electrodes of the tube-like tunnel (11). The resistors between location (20) and (23) increase linearly, the resistors between (23) and (24) have equal resistivity.
The operation starts with an “ion accumulation phase” (A). By a voltage on the order of 300 volts, the steepest electric field profile is generated, producing the highest electric DC field barrier. The ions are driven by the gas flow, symbolically indicated by the arrows (16), against the electric DC field barrier and are stopped there because they cannot surmount the electric field DC barrier. The ions are accumulated on the rising edge of the electric DC field barrier between locations (20) and (23), where ions of low mobility (mainly heavy ions of large collision cross section) collect in the high field near the upper end of the rising edge, whereas ions of high mobility gather in the low field near the foot of the rising edge, as indicated by the size of the dots symbolizing the ions. In a second phase (B), the “trap phase”, the supply of ions is stopped by an attracting voltage at the repeller plate (9), and the ions finally reach their equilibrium locations on the rising edge of the electric DC field barrier. The trap phase is very short in time; the ions assume their equilibrium location in about one millisecond. In a third phase (C), the “scan phase”, the supply voltage of the electric DC field barrier is steadily decreased, and ions of increasing mobility can escape towards an ion detector, particularly to a mass spectrometer operating as ion detector.
The measured total ion current curve presents directly the ion mobility spectrum from low ion mobilities to high ion mobilities. The device is denominated “TIMS”, “trapped ion mobility spectrometer”. Regarding the theoretical basis, see the research article “Fundamentals of Trapped Ion Mobility Spectrometry” (K. Michelmann, J. A. Silveira, M. E. Ridgeway and M. A. Park, J. Am. Soc. Mass Spectrom., January 2015, volume 26, issue 1, pages 14-24).
Improvements of the scan modes for this apparatus have been made to achieve a linear mobility scale, or a constant resolution along the mobility scale (M. A. Park et al., U.S. Pat. No. 8,766,176 B2). Using two of these ion mobility spectrometers in series, mobility filters can be built (M. A. Park et al., US 2012/0273673 A1).
The ion mobility resolution Rmob was found mainly to depend on the scan speed (acquisition speed). The higher the scan speed, the lower the resolution. As already mentioned, ion mobilities of Rmob=250 have been achieved with the small apparatus at slow scans. Since ions generated in the ion source are lost during the scan phases, the utilization rate of ions produced in an ion source, accumulated, and subsequently analyzed in an analyzer, is determined by the ratio q=ta/(ta+ts) wherein ta is the ion accumulation time and ts the measuring scan time, during which no ions are accumulated.
For a certain analytical task, first a required mobility resolution to solve the analytical task has to be chosen, e.g. Rmob≈80. This mobility resolution determines the scan time. With optimum gas pressure and gas speed, the scan time ts to achieve this resolution over a wide range of mobilities amounts to about 60 milliseconds. A simple calculation shows, that for a utilization rate of 80 percent of the ions from the ion source, the accumulation time ta must be in the range of 200 milliseconds. Experience shows, however, that with usual high gain electro spray ion sources, ions get lost in the tube-like tunnel (11) at accumulation times above 40 milliseconds, resulting in a duty cycle of only 40 percent. And it is mainly the most interestingly high mass ions which get lost.
There is still a need for devices and methods operating with highest utilization rates (duty cycle) of the ions generated in an ion source of a mass spectrometer, thereby reducing the restriction of the mobility resolution, in particular with an electrospray ion source coupled to liquid chromatography for analyzing complex samples in the field of bottom-up proteomics.